1. Field of the Invention
The invention relates to a method of storing data on a rewritale data storage medium, to a storage medium, to a recording apparatus for storing data on a rewritable data storage medium and to a playback apparatus for playback of user data stored on a rewritable data storage medium.
2. Description of Related Art
The invention addresses a storage medium on which users can store copyrighted and copy-free material. Often the user has a right to store and copy content, but there are restrictions to the number of (generations of) copies that he can make. Encryption is used to ensure that copy-righted content can only be interpreted by “compliant” devices which adhere to copy protective restrictions. A further protection is needed to avoid that non-compliant devices can make a bitwise copy of encrypted data. This is often avoided by storing essential information, e.g. a decryption key, in a manner that can not be copied.
More generally it is concluded that copy-protective measures require that on recordable discs some data must be stored which shall not be modifiable or erasable by consumer end products. These data will be called “system data” in the following. Examples of “system data” are:
a unique disc identifier number which is used to encrypt the data that the user stores on the disc,
a list consisting of a single key which has been encrypted with a number of different manufacturer-specific or device-specific keys,
a list of electronic serial numbers of revoked devices or revoked discs. By storing such a list on all blank discs, revocation instructions can be disseminated to consumer devices. Upon receipt of such revocation instructions, compliant devices refuse to communicate with revoked devices.
Content or data recorded by the user will be called “user data” in the following. Moreover, the term “fixed data area” will be used for an area of the storage medium in which any information is stored that is read-only and not modifiable by consumer devices. On the contrary, in the “recordable data area” information is stored which can be modified by consumer devices. Also data, which can only be written by consumer devices after some modifications (“hacks”) have been made to the device by malicious users will be stored in the recordable data area. Such modifications can be a change in the firmware or software used to control the recorder.
To store data in the fixed data area requires the use of components which are typically not available in consumer devices. An example of a technique to store such data is a “wobble”, which is a radial deviation of the pit positions or the pregroove from a perfect spiral. Laws of physics and mechanics prohibit that such a wobble can be written on the fly by a laser as available in a consumer recorder for optical discs. Other examples of data stored in the fixed data area are the BCA code, proposed for DVD-ROM, selectively damaged spots on the disc material burned by high power lasers, or data stored in a special area of the disc which contains read-only material.
A practical problem is the storage of large quantities of data in the fixed data area. Typically the capacity is limited to a few (hundreds of) bits. Meanwhile the amount of system data that needs to be stored may well exceed the storage capacity available in the fixed data area.